As optical systems migrate to higher transmission rates, such as 40 Gbs, there is a need to compensate for chromatic dispersion and to optimize residual chromatic dispersion in the system to minimize transmission penalty. Residual dispersion is an artifact of imperfect match between the dispersion in the fiber plant and the fixed dispersion compensators used in typical optical transmission systems. To solve this problem, considerable effort has been devoted to the development of tunable dispersion compensation devices to replace and/or supplement existing fixed dispersion compensation devices. Moreover, as optical transmission systems evolve to more flexible and re-configurable system architectures, there is a need to dynamically compensate for chromatic dispersion as node distances change through reconfiguration or as a result of temperature changes.
To minimize transmission penalty due to chromatic dispersion at high transmission bit rates such as 40 Gbps, close-loop tuning methods are typically used. In the closed-loop method the feedback signal to the controller is correlated to system penalty and the controlled tuning device is a tunable dispersion compensator (TDC). Tuning dispersion in a closed-loop system requires that the device tunes both dispersion and dispersion slope continuously over the complete dispersion range and across all network channels. In available TDCs, as the dispersion is tuned from one value to another, the signal may pass through time periods of unpredictable signal distortion due to uncontrolled dispersion of the TDC before arriving at the desired state. Currently there is no known solution which guarantees avoidance of these periods of additional signal distortion between dispersion setpoints for etalon-based dispersion compensation devices.
There are a number of known approaches to provide tunable dispersion compensation. Technologies includes: Etalons, Fiber-Bragg Gratings (FBG), Arrayed Waveguide Gratings (AWG), Virtual Imaged Phase Array (VIPA), Mach-Zehnder Interferometers (MZI), and Planar Lightwave Circuits (PLC). None of these technologies have produced satisfactory continuous dispersion tuning and/or continuous dispersion slope tuning.